When a temperature of a mold is low in an injection filling step of an injection molding machine, a surface of molten resin filled in a cavity of the mold is rapidly solidified. In this case, a cavity surface of the mold may be insufficiently transferred to a molded product, and a defect referred to as a weld line or a silver streak may occur on a molded product surface.
In order to prevent the defect, a molding method has been proposed in which a heating medium is supplied to a medium passage of a mold during a time between mold opening and completion of resin filling, and a cooling medium is supplied during a time between the completion of resin filling and mold opening in a series of steps including injection filling, pressure retention, cooling, and mold opening and closing. Thus, molten resin is filled in a mold previously heated to a temperature equal to or higher than a thermal deformation temperature of resin to delay solidification of a resin surface, and after the resin filling, the mold can be cooled to a temperature equal to or lower than a glass transition temperature or the thermal deformation temperature of resin before mold opening, thereby preventing the above described defect.
However, taking a long pressure retaining time for a thick molded product or the like may cause a molding defect such as a sink mark if a temperature of a mold is reduced during pressure retention after completion of injection and cooling and solidification of the molded product proceed. Also, increasing a heating temperature of the mold to increase the time for pressure retention may increase a cooling time to reduce productivity and waste heating energy consumed. Further, a low heating temperature of the mold cannot provide a sufficient pressure retaining time.
Some molded products are molded from crystalline resin by injection molding. The crystalline resin is resin crystallized during injection molding. The crystalline resin includes resin with high crystallization speed such as polypropylene or polyethylene and resin with low crystallization speed such as polylactic acid. The crystalline resin can be increased in the degree of crystallization and thus increased in strength, and can be applied to a functional component. Thus, a molding method for increasing the degree of crystallization is much needed.
However, for the crystalline resin with low crystallization speed, the resin sometimes cannot be sufficiently crystallized during injection molding, and temperature control during injection molding has been improved to ensure crystallization (for example, see Patent Document 1).
In the technique described in Patent Document 1, crystallization of crystalline resin is more efficiently performed during temperature increase than temperature decrease, and thus the temperature is reduced to a temperature range or lower where the crystalline resin is crystallized and then the temperature is again increased to promote crystallization of the crystalline resin.
Patent Document 1: Japanese Patent Laid-Open No.